1. Field of Invention
The invention relates generally to antennas used in mobile communication devices, such as cell phones. More particularly, the invention relates to antennas used for near field communication (NFC) and radio frequency identification (RFID).
2. Related Art
As mobile phones become more popular and they are providing more services on different frequency bands. Increasingly, mobile devices have not just a single antenna intended to handle voice communication, but rather a plurality of antennas for various communication services. For example, a mobile phone may include separate antennas for voice and data communication over several GSM cellular bands and CDMA bands. In addition to antennas for bands required for cellular communication, many mobile phones include antennas for Bluetooth® communication with peripheral devices, multiple bands of Wi-Fi and NFC. With added services, antenna complexity increases dramatically. It is becoming increasingly difficult to provide antenna arrangements suitable for supporting operation of all of these services.
In an ideal world, each service could have a dedicated antenna that is designed strictly for that service. It would have antenna characteristics that made it suitable for use in that service and would not radiate at frequencies outside of the intended band of operation. However, it is not practical to include multiple perfectly designed antennas in mobile phones. Some mobile phones have multiple antennas, each intended to support a particular communication service. Sometimes design compromises must be made in the interest of space and form factor that render one or more of the antennas less that “ideal” in the sense that they radiate beyond the intended band. Other mobile phones have single or multiple antennas at least some of which are designed to handle multiple communication services. These services operate on diverse frequencies. Antennas must be designed to radiate in different frequency ranges. This makes them susceptible to becoming activated (by induced currents) to radiate at frequencies not intended, such as, for example, a harmonic frequency of an intended radiation frequency of a neighboring antenna.
The various communication services have different non-linear components associated with them which may cause unintended harmonics to appear which may in turn activate one or more neighboring antennas with the same device.
Alternatively, an antenna system within a mobile or other device may radiate sufficiently at a harmonic or intermodulation frequency that the whole device is close to failing electromagnetic compatibility specifications (EMC).
It is difficult to design an antenna for a small space, such as the space available in a mobile phone that will radiate only frequencies intended to be radiated. Many antenna designs have a wide range of “undesired” frequencies at which they may radiate.
Circuits driving these antennas are often not designed to generate only the exact frequencies desired to be radiated. It is well known that a pure “sine” wave at frequency f1 in the time domain generates only a single frequency f1 in the frequency domain. However, as shown in FIG. 1, a square wave at frequency f1 generates not only frequency f1, but also many harmonics of frequency f1. Driver circuits that are imperfect (it is not practical to build “perfect” circuits that will not generate some undesirable harmonics of desired frequency signals) generate harmonics that may be radiated by antennas even though it is desired that they not be radiated. This is wasteful of energy and can cause interference. It can even cause radiation to occur in violation of energy and spectrum requirements set by various laws and regulations intended to control the radiation spectrum assigned to various classes of wireless services.
What is needed is a simple and cost-effective way to reduce unwanted spurious emissions from antennas of the type commonly used in mobile communications devices, particularly those used for NFC and RFID communications in the 13.56 MHz. frequency band; to do so without substantially affecting radiation characteristics at desired frequencies.
Features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.